1. Technical Field of the Invention
The present invention relates to an electrophotographic image forming apparatus, and more particularly, to an electrophotographic image forming apparatus wherein a developing unit which is provided with a plurality of developing means is capable of performing relative movement to a photoconductor by which each one of the developing means is selectively positioned at a predetermined developing position opposite to the photoconductor.
2. Description of Related Art
As an image forming apparatus of this kind, copying machines and laser printers have heretofore been offered that are provided with four developing devices which contain toner of three primary colors of cyan, magenta and yellow, and black for composing each color image by successively performing developing operation on each color to form a color image. The apparatus is also arranged to be able to perform a monocolor image forming operation under a monocolor mode by using only one optional color or a composite monocolor image forming operation under a composite monocolor mode by composing images with a combination of two colors.
There are two methods for arranging positional relation between each one of the developing devices and the photoconductor in the image forming apparatus, that is, (i) all developing devices are disposed always at a predetermined developing position (the position opposite to the surface of photoconductor) as the one disclosed in U.S. Pat. No. 4,579,443, (ii) only a developing device which performs developing operation is positioned at a developing position and other devices are retracted.
The latter method (ii) includes the following two methods; (a) Rotary Method: Developing sleeves of four developing devices are disposed at regular intervals on the circumferential surface of an imaginary cylinder and by rotating a holding member of the developing device around the central axis of the cylinder as the axis of rotation, one of the developing devices selected is only positioned at the developing position as disclosed in Japanese Patent Application TOKKAI SHO61-151564 and U.S. Pat. No. 4,620,783, and (b) Method for moving up and down: Four developing devices are disposed up and down at regular intervals to compose of an unit, and by moving the unit in the upward-and-downward direction, only one developing device is positioned at the developing position as disclosed in U.S. Pat. No. 4,620,783. As transformed methods of the upward-and-downward moving method as described in (ii) (b) above, it may be considered to arrange the movement in the different directions such as the one which is arranged to move around the photoconductor in reciprocating motion.
However, in the case of the apparatus which is arranged to relatively move the unit with respect to the photoconductor for selective use of the plural developing devices as described in the method of (ii) above, toner replenishment for each one of the developing devices is conducted from the position out of the operating range of the unit for operational convenience' sake. In this case, it is necessary to move the unit to the position where toner can be replenished when toner is replenished to a developing device.
For instance, the image forming apparatuses disclosed in U.S. Pat. Nos. 4,841,329 and 4,620,783 are provided with a detecting means in each developing device for detecting residual toner in order to detect a shortage of toner, and whenever a developing device is detected short of toner, the developing device is always moved to a toner replenishing position. It may also be considered to arrange the toner replenishing operation to be performed when predetermined conditions such as turning on of the main switch of the image forming apparatus, completion of image forming operation and the like are satisfied other than the time when toner shortage is detected.
However, in a copying machine which is provided with four color developing devices of cyan, magenta, yellow and black, copying operation is not necessarily be conducted under a color mode using four colors as described above. For instance, under the monocolor mode, only one selected colored toner is used. Under the composite monocolor mode, only two selected colors (red: magenta, yellow, blue: cyan, magenta, green: yellow, cyan) out of cyan, magenta and yellow are used.
Accordingly, in the control to return the developing unit to the uppermost position when a predetermined condition is satisfied, there arises a problem how to determine predetermined conditions. For instance, for users who frequently conduct copying operation using black toner only, it is a waste of time and energy if the developing unit is controlled to be returned whenever colored toner empty is detected. For users who frequently conduct copying operation using four full colors, there arises a problem that the reproduction of color tone becomes worse unless the developing unit is immediately returned for toner replenishment whenever any one of the colors is detected as empty state. For users who want to conduct copying operation as quickly as possible even if the density of reproduced image is somewhat light, it is preferable not to have the returning operation at all. The conditions to be fixed preliminarily thus differs according to the purposes of use by users. Under the circumstances, an image forming apparatus which can comply with these requirement is eagerly required for.
On the other hand, in the method of (ii) (b) above, it may be considered to arrange the uppermost position of a unit as a toner replenishing position to readily replenish toner from above. And, by arranging the position as a developing position of black developing device which is frequently used and setting the position as a home position, it will become advantageous in changing over developing devices of the unit and the movement of each developing device for toner replenishing operation is rationalized.
In this kind of image forming apparatus, however, the developing device can not be fixedly positioned at the developing position when image forming operation is started and completed since it depends on a developing mode set. For instance, in the case of color mode, developing operation is carried out in a predetermined order of cyan, magenta, yellow and black. Accordingly, the cyan developing device has to be positioned at the developing position when image forming operation is started. Consequently, when the image forming operation is finished, the black developing device is supposed to be positioned at the developing position. In the case of monocolor mode, a specified developing device has to be positioned at the developing position when image forming operation is started, and said developing unit is positioned again at the developing position when the image forming operation is finished. In the case of composite monocolor mode, developing device can not be fixedly positioned when image forming operation is started and finished since it depends on the color specified. Accordingly, in such an image forming apparatus, it is a waste of time and energy to forcibly return the developing unit to a standard position when image forming operation is started and finished. It is, therefore, desired to avoid such an inconvenient returning operation.
Further, in the case of the method of (ii) and its transformed method, the developing unit is provided with large inertia by its own weight, and the state is stabilized since any one of the developing devices is arranged to be positioned at a predetermined developing position when they are not being moved. Accordingly, even if the main power source is turned off by jamming trouble and the like for the sake of safety, any inconvenience is not occurred caused by the developing unit.
However, the following inconveniences might occur if the main electric source is turned off during the movement of the developing unit.
(i) Generally, braking force is given to a moving developing unit from a certain time point for smoothly positioning the unit at the developing position. If the electric source is turned off before such braking force is given, the developing unit is excessively moved by the inertia, and consequently, it may damage the developing unit or peripheral members of the unit.
(ii) Generally, the position of the developing unit is detected discontinuously. In other words, the position of the developing unit can be detected only when any one of the developing device is positioned at the predetermined developing position. Accordingly, if a developing device is stopped when the device is not positioned at the predetermined developing position, the position of the developing unit can not be detected after the machine is reset when trouble, such as a paper jam occurs, and the movement of the developing unit can not be controlled, thereby prohibiting image forming operation.
(iii) If a developing unit which is not positioned at a predetermined position is manually moved in order to remove a jammed sheet of paper, there arises a possibility for damage the developing unit or peripheral members of the unit.